LiFePO4 having an olivine-type (Pnma) crystal structure as a cathode material used in a secondary battery such as a metal lithium battery, lithium ion battery, or lithium polymer battery is subjected to electrode oxidation/reduction accompanied by doping/undoping of lithium during the process of charging and discharging. Such secondary batteries are attracting attention as large capacity batteries in recent years.
Conventionally, the following methods have been suggested as methods for synthesizing the cathode material LiFePO4; (1) A method comprising the steps of mixing ferrous phosphate octahydrate [Fe3(PO4)2.8H2O], ammonium hydrogenphosphate [(NH4)2HPO4], and lithium carbonate (LiCO3) at a specific ratio and calcining the mixture (JP-A-Hei 9-171827, for example); (2) A method comprising the step of mixing iron oxalate (FeC2O4) as an iron source with ammonium hydrogenphosphate (NH4H2PO4) and lithium carbonate (LiCO3) at a specific ratio (JP-A 2000-294238); (3) A method comprising the steps of adding a carbon material to a mixture of Fe3(PO4)2.8H2O and Li3PO4, and calcining the mixture in an atmosphere containing 1012 ppm or less (by volume) of oxygen (JP-A 2002-110163, for example).
Also, (4) a method comprising the steps of mixing Fe3(PO4)2.8H2O and Li3PO4 as ingredients with an organic material (polymer, monomer, low-molecule weight compound, etc.) which is turned into carbon deposits by pyrolysis, and calcining the mixture to cause pyrolysis of the organic material (JP-A 2001-15111) has been suggested.
However, the starting materials used in the above methods (1) to (3) are all secondary compounds which are expensive and difficult to obtain. For example, Li3PO4 and iron oxalate (FeC2O4) are both relatively expensive and cause an increase in the production costs of the cathode material. Fe3(PO4)2.8H2O as another iron compound can be synthesized from Na2HPO4 and Fe(II)SO4.7H2O, for example, but it is a hydrate whose hydration number is unstable and it is therefore difficult to control the feeding of it in a stoichiometric manner. Also, since Fe3 (PO4)2.8H2O is obtained as a precipitate in the synthesis process thereof, a cumbersome process such as filtering is required to remove sodium ions and so on. However, it is difficult to remove sodium ions and so on completely, and such a process may bring the entry of impurities. To carry out the filtering completely to increase the purity of the calcination precursor, it is preferred to allow the crystals of Fe3(PO4)2.8H2O precipitates to grow until they reach a large diameter (about 10 μm or greater). However, when a mixture of Fe3(PO4)2.8H2O particles with a large diameter and Li3PO4 is calcined, the resulting LiFePO4 particles have a large diameter and have low activity as a cathode material.
As described above, the conventional techniques for producing LiFePO4 have problems of the entry of impurities and the necessity of a cumbersome process. Also, since primary materials which are inexpensive and easily available such as metal iron cannot be used, the cost is unavoidably high. Thus, any of the conventional techniques is not satisfactory as a method for producing LiFePO4 in an industrial scale.
It is, therefore, an object of the present invention to provide a method for producing LiFePO4 as a cathode material for a secondary battery reliably from primary materials which are easily available and inexpensive.